1. Field of the Invention
This invention relates to antenna elements; and, in particular, to a stripline fed linear slot array antenna.
2. Disclosure Statement
The initial work on waveguide slot radiators was performed during World War II, as reported later by Watson in "Resonant Slots", IEE Journal, Vol. 93, Part 3A, pp. 747-777, 1946 and in a paper by Stevenson entitled "Theory of Slots in Rectangular waveguides", J Appl. Physics, Vol. 19, pp 24-38, 1948. An early study of stripline fed slots is reported in Strumwasser et al, "Slot Study in Rectangular TEM Transmission line", Hughes Aircraft Company, Report TM 265, January, 1952 and discusses a linear slot array fed by a single continuous TEM transmission line. In the Strumwasser et al study, a sixteen-element linear array of inclined longitudinal slots was designed empirically, neglecting TEM line attenuation and mutual coupling. A fundamental difficulty was encountered with excessive resonant slot lengths, which was only partially solved by loading the slot ends in dumb bell fashion. Since the slot spacing was greater than one-half guide wavelength, this configuration was limited to travelling wave arrays with the main beam scanned at least one beamwidth towards endfire. A 30 dB Chebyshev distribution was used in the design and a 22 dB sidelobe level was achieved. The input VSWR was 1.34:1. The TEM line was dimensioned so that all waveguide modes were cut off, and a plurality of inclined series slots were centered over a straight inner conductor. The resonant slot lengths in this configuration were longer than a half-guide wavelength, necessitating an empirical loading method to resonate the slots. The lengths were still too great to permit a standing wave array design (half-wave spacing), so only traveling wave arrays are studied. The inclined series slot also generated a cross-polarized component in the radiated field, which is equivalent to an efficiency loss. These are some of the problems this invention overcomes.
The use of a stripline corporate feed for a linear slot array was first reported in Sommers, "Slot Array Employing Photoetched Tri-plate Transmission Line", IRE Conv. Rec., Vol. MTT-3, pp. 157-162, March 1955 and has been widely used since that time (see FIG. 1). A corporate power divider network was used in which each divider output line was terminated in a cavity-backed slot transverse to the direction of propagation. The slots were aligned in a collinear array. Variable power divider ratios and branch line lengths were used to control the array amplitude and phase distributions. Mutual coupling effects did not enter into the design. This type of array requires empirical optimization of the cavity dimensions. These are yet other problems this invention overcomes.